Continuous multiple stage crystallizers are commonly used in a number of industries, and they are inter alia used in the precipitation of aluminum hydroxide during the manufacture of aluminum oxide. A modern crystallizer for the precipitation of AL(OH).sub.3 in the Bayer process has usually ten or more tanks in series, and the crystals and residual liquor flow from one tank to the next without any sorting out of the crystals. The stream leaving the last tank is classified, the coarse crystal fraction constituting the product, whereas a suspension containing the finer crystals is returned to the first tank of the series. Such recirculation is done for a double purpose: First, to provide seeds for the commencement of crystallization in the first tank; and second, to increase the crystal size of the recirculated finer crystals. The precipitation in the tanks is effected by maintaining the liquor in supersaturated condition, with each tank successively cooler than the previous tank in the series.
Among the disadvantages of such arrangement, however, are that:
(a) the separation step after the last tank is complicated and expensive, PA1 (b) the distribution of crystal sizes in the product is not as may be desired, which may result in difficulties in the further handling of the product, and PA1 (c) the arrangement is sensitive with respect to process instabilities. PA1 A. The residence time for crystals above a minimum product size can in principle be varied in unlimited fashion for a given crystallizer volume. For a given production rate a reduction of the crystallizer volume is achieved. PA1 B. An improved crystal size distribution in the product is obtained due to the fact that: PA1 C. The operation can be made stable without any substantial degree influencing the size distribution of the product crystals. PA1 D. Special separators to separate fine and coarse crystals are superfluous.
Some embodiments of crystallizers which at least partly counteract these drawbacks are known from the patent literature.
Thus, U.S. Pat. No. 3,607,113 discloses a continuous multiple stage crystallizer for aluminum hydroxide. The stages are connected in series so that a liquid stream containing solely fine crystals is passed from one stage to the next. The discharge from the last stage is passed to a separation tank having two outlets. The first outlet discharges crystal-free liquid for further processing and the second discharges crystals and some liquid. The liquid is recycled to the first stage in the series.
In addition, the suspension in each stage is circulated periodically through a cyclone or another type of separator such that the coarser particles can be separated and the remaining stream returned to the stage from which it came. Only one stage at a time has its suspension recirculated, and this is done only when the crystals constitute 50 percent of the volume of the suspension.
The flow pattern of the process according to this U.S. Patent is neither countercurrent nor co-current for the coarser/finer particles. The finer crystals are not completely excluded from the product because a cyclone cannot remove all fine crystals from the coarse crystals. The process is only partially continuous. It is not known whether instabilities in the process can be buffered and if so, in what way.
From German Auslegeschrift No. 1,107,200 it is known to use multistage crystallization for the manufacture of coarse-grained salt by vacuum-cooling. According to this invention, countercurrent crystallization is used, and the liquid containing finer crystals is transported in the opposite direction to the direction of movement of the coarser crystals. The countercurrent flow is achieved by each stage having a classification tube.
The total liquid stream treated in the process must flow up through the classification tubes of each stage. Thus the total through-flow of each of the single is predetermined and cannot be used for adjustment purposes. As the fine crystals can also not be returned to the preceding stage in the series as fine crystals, it is not possible to obtain a defined distribution of the fine particles between the stages. Thus, it is not possible to obtain a product having a selected product size distribution.
According to the invention, it is advantageous to let finer crystals separate from the liquid which is discharged from the last stage and to mix them into the feed to the first stage. As the product is taken from this stage, the possibility exists that the product will contain fine crystals.
Continuous multistage crystallization is also known from German Pat. No. 888,090. The patent pertains to a process and an apparatus for continuous concentration of ice crystals. The apparatus consists of a number of tanks, and the coarser crystals are passed in countercurrent flow to the solution containing the fine crystals. This is achieved by passing a stream of suspension from each tank to an associated classifying centrifuge or other separator, and passing the coarse crystals to the preceding stage, wherein the liquid containing fine crystals is passed back to the stage from which it came. However, the critical size of the crystals cannot be adjusted during the operation of the apparatus. Moreover, a main stream of liquid corresponding to the feed stream and mixed crystals is passed from one stage to the next. With such apparatus, fine crystals are prevented from following the product crystals.
In addition, in the process of this patent, it is possible to control the residence time in each stage only for the coarse crystals and not for the fine ones. The crystallizer is so constructed because coarse particles carry with them relatively less liquid than do fine particles. The only requirement for the distribution of the crystal size in the product is that the crystals are to be as large as possible.